there is an electrical device that sends an intermittent current to the valve which makes the solenoid vibrate and supposedly you can hear the noise.
dont recall name of device and dont know if it works
They are supposed to be accessible at ground level with a valve box cover.
good luck

Plug a large dimmer-equipped lamp into the same outlet as the valve timer transformer. Set the lamp to about half dim. Take a portable AM radio, tune it to an empty frequency near the lower end, 500kHz-700kHz or so. Hold the radio near the cable that goes to a valve, see if you hear the rat-tat-tat noise. Try with the valve on and off. Find the direction of the radio where that noise becomes loudest. With some luck you can then follow the line unless it's buried too deep in the ground. If you can hear it follow the trail until it stops, that's where your valve should be.

CFL (energy saver bulbs) might work as well, they also create radio noise.

Your "contractor" was a hack! Your zone valves should have been in one location and accessible. One "right way" to set up an automatic system is to bring the water supply to a place fairly near where the controller will be located. A manifold is constructed with as many outlets as you need and the valves attached to them. The whole assembly can be underground, but should be in a box. Plastic boxes are usually used, but one could be fashioned from concrete. Since this puts the valves close to the controller, no great amount of wire is needed. If you are using domestic water, the backflow preventer could be located in the same area or near the tee. The backflow preventer should also be in a box. I'd venture to guess the "contractor" is not licensed, so you likely can come back on him, but it would be worth a try.

Well, sometimes one has to just eat it. The contractors here put the valves under (!) the house so I had to hack a hole into the side of the house because the contortion-style crawl jobs were killing my lower back. They must have assembled it before the floors were laid, on the dirt, then pulled it all up. Meaning the defective valve could not be unscrewed because its top hit a beam. Great. To add insult to injury they screwed the brass shut-off valve into a brass tee using a steel nipple. The latter I've seen quite a bit around here, even on jobs done by licensed guys.

But this happened way before we even knew this town, more than 35 years ago so the guy most likely won't even be around anymore. What can you do other than fix the mess?